The present invention relates to a miniature image-capturing lens which is suitable for an imaging device employing a solid-state image-capturing element such as a CCD type image sensor, a CMOS type image sensor and the like.
In recent years, due to the higher performance demands and miniaturization of imaging devices employing solid-state image-capturing elements such as a CCD (a charged coupled device) type image sensor, a CMOS (a complementary metal oxide semiconductor) type image sensor and the like, mobile phones or personal computers employing such imaging devices have become popular. Further, continual miniaturization of image-capturing lenses loaded on the imaging devices is further demanded.
A two-piece structured lens, which makes it possible to develop higher performance compared to a single lens, is suitable for the image-capturing lens for the above-mentioned usage. Well known is a retro-focus type image-capturing lens, composed of a first lens having a negative refractive power, a diaphragm, and a second lens having a positive refractive power, which are arranged in the order from the object side. The image-capturing lens composed as mentioned above is disclosed in TOKKAI 2000-321489 and TOKKAI 2001-183578.
Though an image-capturing lens of this type is suitable for a wide angle function, its back-focus is apt to be long, which makes it very difficult to have a total image-capturing lens length (a distance from a surface located closest the object side in the entire image-capturing lens to an image-side focal point, provided that in the image-capturing lens in which the aperture diaphragm is located closest to the object side, the total length of the image-capturing lens is a distance from the aperture diaphragm to the image side focal point) to be short.
The present invention has been attained in view of the above-mentioned problems. The object of the invention is to provide a miniature image-capturing lens which is smaller and is optimally corrected for aberration, though it simply consists of two separate lenses.
Concerning the size of the miniature image-capturing lens, the present invention aims at the miniaturization of the image-capturing lens to a level satisfying the following formula. By satisfying this limit, the total length of the image-capturing lens is shortened, and its external diameter is synergistically shortened. Therefore, the imaging device can be totally miniaturized and light in weight.
L/2Y less than 1.50xe2x80x83xe2x80x83Formula (5)
Where,
L: distance on an optical axis between the aperture stop and the image side focal point, and
2Y: diagonal length of an effective image area.
Image side focal point means the image point where parallel incident rays, parallel to the optical axis of the lens, are brought to focus. Further, when a plane-parallel plate is arranged between the image side surface of the image-capturing lens and the focal point, the thickness of the plane-parallel plate is treated as an air-converted distance. Here, in the case as one example that the parallel plate section has a low pass filter, the air-converted distance Dc is obtained by the following formula: Dc=t/n, where t is a thickness of the low-pass filter and n is the refractive index of the low-pass filter.
Item 1
A miniature image-capturing lens is composed of:
an aperture stop,
a first lens, having a positive refractive power, whose convex surface faces to the image side, and
a second lens, formed to be a meniscus shape, whose concave surface faces to the object side, and
all of which are arranged in the above order from the object side,
wherein the first lens and the second lens each have at least an aspheric surface, and the following formulas are satisfied:
0.50 less than f1/f less than 0.80xe2x80x83xe2x80x83Formula (1)
0.30 less than (R2+R1)/(R1xe2x88x92R2) less than 1.20xe2x80x83xe2x80x83Formula (2)
where
f1: the focal length of the first lens,
f: the focal length of the total image-capturing lens,
R1: the radius of curvature of the object side surface of the first lens,
R2: the radius of curvature of the image side surface of the first lens.
In order to obtain the miniature image-capturing lens whose aberrations are optimally corrected, the fundamental construction of the present invention is composed of an aperture stop arranged in a total image-capturing lens system at the side nearest to the object, the positive lens with the convex surface facing the image side having a larger refractive power than the object side surface, and the second lens of a meniscus shape facing its concave surface to the object side.
By arranging the aperture stop in the image-capturing lens system at the nearest side to the object, and by having the larger positive refractive power on the image side surface of the first lens, it is possible to keep an exit pupil at a further distance from the image surface. With this arrangement, main light rays of a light flux emitted from a lens final surface becomes to be incident with an angles near perpendicular onto a solid-state image-capturing element. That is, a image side telecentric characteristic necessary for an image-capturing lens used for the solid-state image-capturing element can be secured well and a shading phenomena on a peripheral section of an image screen can be reduced. Therefore, it becomes possible to obtain a structure by which an image side telecentric characteristic is easily secured, which is necessary for miniature image-capturing lens employed in solid-state image-capturing elements. Further, by having such large negative refractive power on the object side surface of the second lens, it becomes possible to correct for any aberration.
Still further, by featuring at least one aspheric surface on each the first lens and second lens, it is possible to correct for aberration optimally. By featuring an aspheric surface on the first lens having the positive power, spherical aberration and coma are effectively corrected for. On the other hand, since the second lens is arranged nearest to the image side away from the aperture stop, there is a difference between the passing height of light flux on the axis and the passing height of light flux off the axis on the periphery area of the picture surface, and by using the aspheric surface, aberration on the peripheral area of the picture surface such as field curvature and distortion, can be optimally corrected for.
Explanation of Formula (1)
Formula (1) shows the conditions by which the refractive power of the first lens is appropriately determined. If the value of f1/f is greater than the lower limit, the refractive power of the first lens can be controlled within the required level, therefore high ordered spherical aberration, coma, and chromatic aberration of magnification are controlled to be lower, which occur on the image side surface of the first lens. If the value of f1/f is less than the upper limit, the positive refractive power of the first lens is moderately secured, and the total length of the image-capturing lens can be shortened.
Explanation of Formula (2)
Formula (2) shows the condition by which a shape factor of the first lens can be appropriately established under the condition of formula (1). If the value of (R2+R1)/(R1xe2x88x92R2) is greater than the lower limit, the image side surface serves mainly as the positive refractive power of the first lens, therefore the image side telecentric characteristic of the total image-capturing lens system is easily obtained. Further, if the above-mentioned value is less than the upper limit, the radius of curvature of the image side surface of the first lens does not become excessively small, and the back focus of the total image-capturing lens system is easily obtained. Therefore it is preferable from the point of view of manufacture of the first lens. It is more preferable to satisfy the following formula (2xe2x80x2).
0.40 less than (R2+R1)/(R1xe2x88x92R2) less than 0.90xe2x80x83xe2x80x83Formula (2xe2x80x2)
Item 2
The miniature image-capturing lens is characterized by satisfying the following formula.
xe2x88x920.50 less than R3/((N2xe2x88x921)xc2x7f) less than xe2x88x920.10xe2x80x83xe2x80x83Formula (3)
where,
R3: the radius of curvature of the object side surface of the second lens,
N2: refractive index of the second lens for d line light, and
f: focal length of the total image-capturing lens.
Explanation of Formula (3)
Formula (3) shows the condition by which the field curvature is easily corrected and the image surface is made flattened, by establishing the negative refractive power of the object side surface of the second lens. The focal length of the object side surface of the second lens is calculated by R3/(N2xe2x88x921) with radius of curvature (R3) and refractive index (N2), therefore formula (3) shows the ratio of the focal length of the object side surface of the second lens to the focal length of the total image-capturing lens.
If the value of R3/((N2xe2x88x921)xc2x7f) is greater than the lower limit of Formula (3), the negative refractive power of the object side surface of the second lens can be controlled within the required level, therefore coma flare of the off-axial light flux and pin-cushion distortion can be controlled not to occur, and an excellent quality image can be produced. If the above-mentioned value is less than the upper limit, the negative refractive power of the object side surface of the second lens is secured, and therefore, the positive Petzval sum decreases, which allows for easy correction of field curvature. Still further, chromatic aberration of magnification, which occurs on the image side surface of the first lens, can be optimally corrected. It is more desirable to satisfy the following formula.
xe2x88x920.40 less than R3/((N2xe2x88x921)xc2x7f) less than xe2x88x920.20xe2x80x83xe2x80x83Formula (3xe2x80x2)
Item 3
The miniature image-capturing lens is characterized by satisfying the following formula.
25.0 less than v1xe2x88x92v2xe2x80x83xe2x80x83Formula (4)
where,
v1: the Abbe number of the first lens, and
v2: the Abbe number of the second lens.
Explanation of Formula (4)
Formula (4) shows the condition by which chromatic aberration of the first lens of a positive power and that of the second lens of a negative power are corrected. If the value of v1xe2x88x92v2 is greater than the lower limit, axial chromatic aberration and chromatic aberration of magnification can be corrected and balanced well.
Item 4
Miniature image-capturing lenses are characterized in that the first lens and the second lens are formed of a plastic material. Here, the description that the lens is formed by a plastic material includes the case that the plastic material is used as a basic material and a coating process is conducted for the surface of the base material for the purpose of the reflection prevention and the surface hardness enhancement.
In recent years, since the miniaturization of the total imaging device is aimed for the objective, developed are image-capturing elements aligned at a smaller pitch, when compared to image-capturing elements having an identical number of the picture elements, that is, developed are the solid-state image-capturing element featuring a light receiving section (a photoelectric conversion section) whose picture surface is smaller. Concerning the image-capturing lens used for solid-state image-capturing elements having smaller picture surface, in order to secure identical image angle, the focal length of the total system must be short, and thereby, the radiuses of the curvature of the each lens and their external diameters can become extremely small. It is very difficult to produce such glasses lens by typical polishing. However, if the above-mentioned first lens and second lens are composed of plastic which can be produced by injection molding, it is possible to perform mass production, even though image-capturing lenses have a very small radius of curvature and a very small diameter. Further, it is relatively easy to make an aspheric surface, to correct for aberration. Glass-molded lenses can also be considered as image-capturing lenses which can be easily produced, though the diameter is too small. However, there is a problem of durability in the metal mold die using the present technology, and therefore, plastic lenses are more suitable for mass production, by which product cost can be limited.
Item 5
Miniature image-capturing lenses are characterized in that the first and second lenses are formed of plastic material whose saturated water-absorption ratio is not greater than 0.7%.
The saturated water-absorption ratio of the plastic lenses is greater than that of glass lenses. When the humidity changes rapidly, abnormal distribution of water absorption occurs in the plastic lenses, causing the refractive index to not be uniform, resulting in a tendency for excellent focusing performance to be lost. Accordingly, by using plastic material whose saturated water-absorption ratio is not greater than 0.7% for the first and second lenses, it is possible to control such deterioration of the performance of the miniature image-capturing lens, caused by temperature change.
Item 6
An image capturing unit is characterized in that the image capturing unit comprises:
a solid-state image capturing element provided with a photo-electrically converting section;
the image capturing lens described in Items 1 to 5 and to form an image of an object on the photo-electrically converting section of the solid-state image capturing element;
a base board to support the solid-state image capturing element and having a terminal for an external connection to transmit and receive electric signals; and
a casing made of light shielding member and having an opening section for incident light coming from the object,
wherein the solid-state image capturing element, the image capturing lens, the base board and the casing are constructed in a single body and the height of the image capturing unit in the direction of an optical axis of the image capturing lens is 10 mm or less.
By using the image capturing lens described in Items 1 to 5, it becomes possible to obtain the image capturing unit having an advantage capable of being made in miniature and realizing high quality image. Here, xe2x80x9copening section for incident light coming from the objectxe2x80x9d is not necessarily limited to an opening to form a space such a hole and may means a section in which a region to allow the incident light coming from the object to transmit is formed.
The description xe2x80x9cthe height of the image capturing unit in the direction of an optical axis of the image capturing lens is 10 mm or lessxe2x80x9d means a total length of an image capturing unit provided with all the above structures in the direction of the optical axis. Therefore, for example, in the case that the casing is mounted on the obverse side of the base board and electronic components are mounted on the back side of the base board, it is supposed that a distance from a tip end of the casing at the object side to a tip end of an electronic component protruded form the back side is 10 mm or less.
Item 7
A mobile terminal is characterized by being provided with the image capturing unit described in Item 6.
By providing the image capturing unit described in Item 6 on the mobile terminal, it becomes possible to obtain the mobile terminal capable of being made in miniature and capturing a high quality image.